This proposal is concerned with determining the three-dimensional structures of a series of related proteins which participate in biological nitrogen fixation. X-ray crystallographic methods will be used in the analysis. The organism to be studied as Azotobacter vinelandii, an aerobic asymbiotic, nitrogen-fixing bacterium. The proteins of interest participate directly in nitrogen fixation or closely related electron transfer reactions. Biochemical procedures for purifying nine of the proteins and one peptide cofactor are known. Large, single crystals of three of the proteins, non-heme iron protein III, flavodoxin, and ferrocytochrome C5, give high resolution X-ray diffraction data and the unit cells and space groups have been derived. Two other crystals forms of non-heme iron protein III, and microcrystals of nitrogenase component I, cytochrome C4 and ferricytochrome C5 have been grown. The specific aim of the research is to solve the high resolution structures of ferrocytochrome C5, flavodoxin and non-heme iron III protein.